Artificial cartilage performs better than the real thing

The smooth cartilage that covers the ends of long bones provides a level of lubrication that artificial alternatives haven’t been able to rival – until now. Researchers say their lubricating layers of “molecular brushes” can outperform nature under the highest pressures encountered within joints, with potentially important implications for joint replacement surgery.

With every step we take, bones at the knee and hip rub against each other. That would quickly wear them away if it wasn’t for the protection afforded by the thick layer of smooth and slippery cartilage that covers their ends.

No amount of polishing can remove all of the small imperfections from the stainless steel used in artificial joints. Any raised areas that are left grind against each other and release debris particles that soften the bone, explains Jacob Klein at the Weizmann Institute of Science in Rehovot, Israel.

Like bone, artificial joints must be covered with a cartilage-like layer. However, while it’s possible to match cartilage’s slick properties at low pressure, at the high pressures found in joints synthetic alternatives “seize up”.

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Under pressure

Now Klein has discovered a possible solution. Working with colleagues in the UK, he’s developed molecular brushes that slide past each other with friction coefficients that match those of cartilage. In some respects, they perform even better&colon; the brushes remain highly effective even at pressures of 7.5 megapascals. Cartilage performs well only up to around 5 megapascals – a natural limit because joint pressure only rarely exceeds that level.

Each 60-nanometre-long brush filament has a polymer backbone from which small molecular groups stick out. Those synthetic groups are very similar to the lipids found in cell membranes, says Klein – although they’re neutral overall, they are positively charged at one end and negatively charged at the other.

In a watery environment, each of these molecular groups attracts up to 25 water molecules through electrostatic forces, so the filament as a whole develops a slick watery sheath. These sheathes ensure that the brushes are lubricated as they rub past each other, even when firmly pressed together to mimic the pressures at bone joints.

Stronger bonds

There’s another reason for the huge improvement in performance, however. The previous generation of brush filaments were weakly anchored by shared water-repelling properties with the artificial joint surface below – a sheet of the mineral mica in these simulations.

When two of the mica sheets were rubbed against each other under pressure, the filaments simply tore away from the mica surface and their lubricating properties were lost. In the latest experiments, the mica is first coated with a chemical to encourage polymer growth. The team can then grow the filaments directly from the mica surface and each forms strong covalent bonds to the surface that are less likely to shear away.

Jennifer Elisseeff, a professor of engineering and orthopaedic surgery at Johns Hopkins University in Baltimore, who was not involved with the study, says the new material is an “important step forward” for joint lubrication studies.

Bone damage risk

Farshid Guilak, a professor of orthopaedic surgery at Duke University in North Carolina agrees. “It is very exciting to see that artificial polymer brushes can be designed to provide such low frictional properties,” he says.

However, Elisseeff and Guilak both wonder how well the brushes will perform in the real world. Guilak points out that Teflon was initially used in joints, thanks to its low-friction properties, but proved to be a “disaster” because it wore out rapidly to leave bone-damaging debris.